One of the primary applications of production logging is to determine oil and water flow rates at various depths in a well. These rates are calculated by measuring values of fluid properties such as oil, water and gas velocity, density and capacitance. The accuracy of these measurements is suspect and has great impact on the accuracy of the calculated downhole flow rate.
In production logging, fluid velocity is usually measured with a "spinner-type" flowmeter. The spinner is calibrated by passing the tool through a fluid-filled wellbore at a constant speed. By successively recording the resulting spinner rotational speed and the corresponding depth location, a continuous flow-survey or fluid velocity log will be obtained. Using this survey, flow rates in the wellbore at different depths can be readily determined to prepare a representative flow profile of that well. Even though spinners have been widely used for many years and have been greatly improved, they still have many disadvantages and restrictions.
Some disadvantages of spinner flowmeters are created by mechanical problems, while others are created by the properties of the fluid and the flow which is being measured. For example, the impeller of the spinner rotates on a bearing which wears and requires frequent inspection and replacement to keep frictional effects from influencing the measurements. Additionally, the spinner requires calibration which must be done downhole, necessitating multiple logging runs at various speeds. In reference to fluid properties, the spinner speed is not only affected by changes in velocity of the fluid, but also by changes in fluid viscosity, flow regime, fluid density, and temperature and pressure.
Furthermore, fluid properties in general have a substantial impact on the accuracy of all production log-derived profile techniques, especially in the measurement of multiphase fluid flow. Quantitative analyses of these multiphase flows are extremely vulnerable to error. For example, spinner type flowmeters, as described above, and basket type flowmeters, while functioning well in single phase flow, are ineffective in multiphase flow due to the flow regimes inherent in such flows. These devices may be calibrated for operation in a two phase flow environment, however, such calibration cannot accurately compensate for actual flow regimes encountered in the field. Capacitance probes, used to determine the holdup fraction in gas-liquid, or liquid-liquid type flows, thereby increasing the effectiveness of the above mentioned devices, are reliable only in wells producing with watercuts under 50%. Additionally, the capacitance probe is further limited by the fact that it is unable to distinguish oil from gas, due to the variation in dielectrics. Similarly, nuclear fluid density tools, also used to increase the effectiveness of spinner and basket type devices, fail due to the inability to effectively distinguish between oil and water. Moreover, while density and capacitance tools can be calibrated for flow regime and fluid type, again such calibration will not accurately compensate for actual flow regimes encountered in field use. Additionally, once the above measurements are made, two correlations must be used to calculate individual gas, oil, and water flow rates. The accuracy of these correlations is suspect which leads to further deviation from true flow rate values. Therefore there exists in the industry a need for a simpler, more accurate method for measuring bottomhole production rates.